Add optional argument to pass gflow in `generate_from_graph()`

[pafloxy]

Is your feature request related to a problem? Please describe. NA

Describe the feature you'd like Currently the facility to generate a Pattern from a graph is provided by the function generator.generate_from_graph() which takes as arguments the following generate_from_graph(graph, angles, inputs, outputs, meas_planes=None) . I think it would be good if we can have an option to pass a gflow of our choice rather than invoking the find_gflow() internally.

Something of the form generate_from_graph(graph, angles, inputs, outputs, meas_planes=None, set_gflow= None) would be convenient. Incase None is passed it should proceed as usual.

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